The present invention relates to photovoltaic cells. Considerable effort has been devoted in the art heretofore to development of photovoltaic cells, i.e., semiconductor devices that can convert light into electrical energy. Typically, such cells incorporate multiple layers of semiconductor materials including n-type semiconductor, where the predominant or majority charge carriers are electrons, and p-type semiconductor, in which the majority charge carriers are holes. These layers cooperatively define a p-n junction. Electrodes are provided in contact with the semiconductor materials on opposite sides of the junction. When isolated from one another, p-type material and the n-type material have different Fermi levels due to differing doping. The Fermi level is an energy level such that the probability of the level being filled with electrons is 50%. When the p-type and n-type materials are united with one another at the p-n junction of the cell, the Fermi levels come into equilibrium with one another and a space charge region forms. The space charge region provides an electric field in the vicinity of the junction. As light impinges on the semiconductor material, incoming photons cause electrons to be promoted from the valence band of the semiconductor material to the conduction band, thus creating an increased number of charge carriers. The term “band gap” refers to the difference in energy between the valance band of a semiconductor material and the conduction band of that material.
The electric field of the space charge region accelerates the charge carriers across the p-n junction. The holes and electrons move in opposite directions. The electrons pass to a first electrode in contact with the n-type material, whereas the holes pass towards a second electrode in contact with the p-type material. This creates a difference in electrical potential between the electrodes and thus creates useful, available electrical energy at the electrodes. An external circuit connected to the electrodes can utilize this electrical energy.
The voltage or potential difference available from such p-n junction cells is limited. The maximum voltage output from such a cell is limited by the difference between the energy level of the conduction band in n-type material and the energy level of the valence band in the p-type material. This difference typically is less than the band gap of the semiconductor. It is desirable to form photovoltaic cells from materials having wide bandgaps as, for example, about 1.7 electron volts or more. Wide bandgap materials can efficiently absorb light at wavelengths of about less than 800 nanometers. Such light is in the visible and ultraviolet portions of the spectrum and constitutes a substantial portion of the solar energy impinging on the Earth. Moreover, cells formed from wide bandgap materials can be used in conjunction with cells formed from narrow bandgap materials. In such an arrangement, the wide bandgap cell is disposed in front of a narrow bandgap cell. Long wavelength light is not absorbed by the wide bandgap cell and passes through to the narrow bandgap cell, where it is absorbed.
p-n junction cells formed from silicon can be made by relatively inexpensive processes such as dopant implantation into silicon wafers. However, silicon has a bandgap of 1.12 eV. Manufacture of p-n junction cells in certain wide bandgap semiconductor materials requires formation of multiple layers by a sequential process of epitaxial deposition. In an epitaxial deposition process, the material is grown on a substrate by depositing materials, most typically from a vapor or gas state, on an existing solid crystal, so that the grown crystal forms in a structure having crystal lattice spacing determined by the lattice spacing of the substrate. However, it can be difficult to grow high-quality semiconductor materials of opposite conductivity types using certain wide bandgap semiconductor materials. Thus, despite the considerable effort in the art heretofore to development of photovoltaic cells, further improvement would be desirable.